Why do CSS styles break?

Hello at any time of the day!
Could you please help me with a small issue?
I've been working on optimizing the site. I'm writing styles, and everything works well. The styles are displayed. But within half an hour, they somehow disappear. Everything is written in the files, nothing has been deleted. I've cleared the cache and added !important, but nothing seems to work.
What should I do in this case?
The theme is AT-Commerce, if that’s important.
Thank you in advance for your response!